A DC/DC converter is frequently used in applications for lowering or raising the output voltage of a DC power source to obtain a predetermined direct current output voltage, for example, a power circuit for a mobile phone. The DC/DC converter turns a switching element on/off, and controls the on/off period in this case, thereby converting a direct current input voltage into a predetermined direct current output voltage. MOSFET is generally used as the switching element.
This type of DC/DC converter has a coil. Thus, there is need to form a closed circuit for releasing electrical energy accumulated in the coil during the off-period of the switching element. Previously, the closed circuit was formed by a free-wheeling diode.
When the free-wheeling diode was used, however, its forward voltage drop was relatively great, thus posing the problem that power consumption at this portion aroused a decrease in the efficiency of the DC/DC converter.
Under these circumstances, a proposal has been made for a DC/DC converter which uses MOSFET being a switching element instead of the free-wheeling diode, and utilizes the switching function of the MOSFET itself to curtail the above-mentioned forward voltage drop. Incidentally, compared with the forward voltage of the free-wheeling diode, voltage loss due to the resistance of MOSFET in the ON-state is small, and power loss is diminished correspondingly.
Incidentally, in the DC/DC converter in which the freewheeling diode has been replaced by the switching element, two of the switching elements composed of MOSFET's are connected to each other in series. That is, the switching element for converting the output voltage to a desired value (hereinafter referred to as a main switching element), and the switching element for releasing energy accumulated in the coil during the OFF-period of the main switching element (hereinafter referred to as a subordinate switching element) are connected to each other in series A step-down DC/DC converter, for example, is constituted such that a direct current output voltage is withdrawn from the point of connection between both switching elements via the coil.
In this type of DC/DC converter, however, the following harmful phenomenon may arise: When, in a mode where the main switching element is in the OFF-state, and the subordinate switching element is in the ON-state, the subordinate switching element is switched to the OFF-state, an electric current based on the energy accumulated in the coil flows into the DC power source via a parasitic diode of the main switching element.
In order to avoid such a harmful phenomenon, in this type of DC/DC converter according to the prior art, it is common practice to monitor the coil current flowing through the subordinate switching element, detect a point in time at which the coil current becomes zero, and switch the subordinate switching element to the OFF-state at the time of the detection. This is because the harmful phenomenon occurs after the coil current becomes zero.
Thus, in the DC/DC converter according to the prior art, which has the main switching element and the subordinate switching element and turns these switching elements on/off alternately to obtain a predetermined direct current output voltage, a current detection circuit is provided in order to detect the above coil current (particularly, its direction). Such a current detection circuit can be realized, for example, by utilizing the ON-state resistance of the subordinate switching element, or by connecting in series a resistor for current detection, and comparing voltages at both ends of this resistor by a comparator for the purpose of monitoring. That is, when the voltage between both ends of the resistor has become zero, it can be detected that the coil current, which is the object to be detected, has become zero.
The following patent documents exist as publicly known documents disclosing DC/DC converters each of which has a main switching element and a subordinate switching element and turns these switching elements on/off alternately to obtain a predetermined direct current output voltage.
Patent Document 1: Japanese Utility Model Registration No. 2555245
Patent Document 2: Japanese Patent No. 3637904
The above-mentioned current detection circuit uses, as a current detection means, the current detection resistor of a very low value, or the ON-state resistance of the subordinate switching element. Thus, the level of the detected voltage is so low that the point in time at which the current has become zero needs to be detected promptly at a high speed and with high accuracy. Therefore, if the comparator is included as stated above, for example, it is necessary to use a comparator having a high speed and high accuracy. Not only is this comparator itself costly, but a high speed, high accuracy current detection circuit has to be constructed. A high drive current is required for the current detection circuit, and presents an impediment to downsizing or efficiency improvement of the DC/DC converter. If the resistor is included in series with the switching element as the current detection circuit, a power loss is caused by the resistor itself, becoming an obstacle to realization of high efficiency. Moreover, the level of the detection voltage is so low as to make a quick response difficult. Thus, actions may be rendered unstable by deviation due to offsetting of the comparator or a response delay. These are fatal drawbacks, particularly when the DC/DC converter is formed into an IC chi configuration.
The present invention has been accomplished in the light of the above-described conventional technologies. It is an object of the invention to provide a DC/DC converter which alternately turns on/off a main switching means and a subordinate switching means to obtain a desired direct current output voltage, which can eliminate a harmful phenomenon ascribed to the inversion of the polarity of a current flowing into a coil, and can also contribute to efficiency improvement, downsizing including IC chip formation, and cost reduction.